German patent document DE 10 2007 020 503 A1 discusses a hydraulic vehicle braking system, which is equipped with two brake circuits, which can be actuated via a shared master brake cylinder by means of a brake pedal of the vehicle. Each brake circuit acts on two wheel brake units at the vehicle wheels. The vehicle braking system has a hydraulic pump unit having a separate hydraulic pump for each brake circuit, and a shared electric motor which actuates both hydraulic pumps. Driver assistance systems such as an electronic stability program ESP can be supported via the control of the pump unit.
In hydraulic braking systems which are used as drive source in hybrid vehicles having an electric motor and an internal combustion engine, there arises the problem that an electrical brake torque that is acting on the vehicle is produced in recuperation or generator operation of the electric motor, which must be taken into account in the braking process when actuating the mechanically acting wheel brake. Since the recuperative brake is available only if the electrical energy store is not fully charged, brake situations with and without electrical brake torque may arise, depending on the charge state of the electrical energy store. Since the brake torques stemming from the different brake devices have a cumulative effect on the vehicle, the lack of or the connection of an electrical brake torque must be compensated by the driver by a modified mechanical brake torque, which requires a change in the brake pedal setting.